Battery Life Calculations

As a battery discharges, its voltage drops. When its voltage drops too low, the electronics it is powering will stop operating. This section explains how to estimate "how long a battery will be able to power your flight computer".

Batteries Powering Flight Computer CPU & Logic

Most flight computers will operate until the voltage of their CPU battery drops below 80% of the rated voltage. For a 9-volt system, this means the computer will operate down to about 7 volts. Some electronic technologies can operate down to 50% or less of their rated voltage. Read your flight computer's specifications. If the specification states a minimum supply voltage, then use that number as the limit. (If it's not in the specification, flight computer manufacturers will likely respond to an e-mail asking this question.) Otherwise, use the 80% value for the limit. (G-Wiz states that their LCX and MC2 computers will operate reliably down to between 6.5 and 7 volts.)

The below chart is extracted from the Duracell MN1604 Alkaline battery datasheet (Duracell.com web site).

Let's assume that the flight computer draws 100mA of current while operating. (The GWiz MC2 draws 100mA while flashing and beeping.) This chart shows that, under these conditions, a typical battery will discharge down to 7 volts in about 2.3 hours. The value is read from the Service Hours axis at the point where the 7.0V line crosses the 100mA line.

The 2.3-hour service life is for the typical battery. Some will last longer. Some won't last this long. A battery will also discharge faster at the higher temperatures of the Black Rock Desert. To be safe, this battery powering a 100mA load, should be replaced after 1.5 hours. (I used an arbitrary 35% derating factor to account for increased operating temperature and variation from "typical" life.)

Also note that the charge in a battery is reduced over time, even if it’s not connected to anything. When stored at room temperature, the typical alkaline battery loses about 5% of its capability in the first year (3% in subsequent years). Storing batteries at high temperatures will deplete them faster.

NiMH rechargeable batteries will lose about 30% of their charge for each month that they are stored. If you are using rechargeable batteries, they should be recently charged. Deeply discharging a rechargeable battery may permanently damage it. To prevent over discharging, these batteries should be recharged before they use 80% of their calculated ampere-hour capacity. Under good care conditions a NiMH battery can be recharged over 500 times.

Depending on how long your rocket sits armed on the launch pad and how long it takes you to recover it and turn off the computer, a battery may only last one flight. Rechargeable batteries will typically discharge faster than equivalent single use alkaline batteries.

Trick for reading log/log charts:
The previous chart uses logarithmic scales on both axis The gridlines across the bottom of the chart are read as 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100 mA. If the chart continued, the next values would be 200, 300,400,... This numbering also applies to the vertical axis.

Note that the #3 grid line is about half way between 1 and 10. Note also that the 7V line crosses the 100 mA line at a Service Hours value half way between 2 and 3. This halfway point corresponds to the value of 2.3 hours.

Batteries Powering Pyro Circuits

The discharge characteristic of a pyro battery is very different. When a flight computer is turned on and waiting for launch, pyro battery power is used to repeatedly test e-match electrical circuit continuity. Then, when a pyro channel fires, a significant amount of current is drawn from the battery for a short period of time.

During continuity testing the average drain on the battery is approximately 2 milliamps. From the above chart, it can be seen that the battery should last over 200 hours in this mode.

During a pyro event an e-match will attempt to draw 9 Amps from the 9V battery. The battery is unable to supply this much current and the internal resistance of the battery will limit its current to about 3 Amps. These batteries are not designed to deliver currents in this range and there is nothing in their specifications to describe their behavior. So data must be obtained by experimentation with a specific brand/model of battery.

The construction of e-matches also varies greatly. When exposed to 9V, some e-matches pop immediately, others last a couple seconds before they burn through. The e-matches that pop immediately seem to have very little drain on the battery’s capacity. A single battery has been used over 7 times to ignite e-matches of this type without any measurable decrease in the battery’s voltage. When a battery delivers a high current, localized internal heating occurs. Prolonged overheating of this type will damage the battery and could cause it to explode. It is strongly suggested that you test the battery/e-match combination that you are using.

To test your battery/e-match combination:

1. Measure your battery’s voltage.
2. Use the battery to ignite an e-match.
3. Allow the battery time to dissipate any heat that was internally generated.
4. Repeat the above steps.

You can either repeat the above steps until you notice that the battery is having trouble igniting the e-match, or you decide that you don’t want to burn any more e-matches in the experiment. In either case, use half the number of igniters burnt as your limit.

It is recommended that you do not power your flight computer with the same battery that is used for pyro events. When a large current is drawn from a battery, its voltage temporarily reduced. (The voltage at the terminals of a 9V battery could easily drop to 4V while powering a pyro event.) This voltage drop could well be too low for too long for your flight computer handle. If your flight computer is deprived of power, it will behave as if it was turned off and back on. It will forget that the launch event occurred and probably fail to trigger subsequent pyro events. Memory inside the flight computer may also be corrupted.

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